It is known that the surface waters of solid masses of granite generally have a low mineral content. In addition, agriculture contributes towards increasing the nitrogenous pollution of surface waters. The result of this is that water treatment specialists are confronted with a twofold problem:
remineralizing the water before it is distributed in the drinking water system, and PA1 removing the nitrogenous pollution in order to meet the quality standards for water intended for human consumption. PA1 as regards the remineralization of water, this can be carried out chemically, for example by adding lime and CO.sub.2 ; PA1 as regards the removal of the nitrogenous pollution, physicochemical or biological processes can be used.
Each of these problems can be solved separately using standard treatments:
The physicochemical processes do not remove the nitrates, they only move them: they are found in the regeneration eluates which also contain high concentrations of chlorides, or other regeneration salts; in addition, these processes are sensitive to the presence of organic materials in the water to be treated.
The biological processes are well suited to the removal of nitrogenous compounds; however, they do not allow colloidal substances to be removed. Moreover, these processes are dependent upon the temperature of the water to be treated and they have hitherto been used industrially only for the treatment of drilling waters having a constant temperature above 10.degree. C.